Fabrication method for organic semiconductor transistor having organic polymeric gate insulating layer

ABSTRACT

Provided is a method for fabricating an organic semiconductor transistor having an organic polymeric gate insulating layer. The method includes forming an organic gate insulating layer on a substrate by a vapor deposition method using organic monomer sources, and causing a polymerization reaction to occur in the organic gate insulating layer to complete an organic polymeric gate insulating layer. Since the vapor deposition method, which is a low-temperature dry-type technique, is employed, the organic polymeric gate insulating layer can be uniformly formed on a large-area substrate by a simplified in-situ process.

TECHNICAL FIELD

The present invention relates to a fabrication method for a transistor,and more particularly, to a fabrication method for an organicsemiconductor transistor having an organic polymeric gate insulatinglayer.

BACKGROUND ART

In order for a material to be used as a gate insulating layer in anorganic semiconductor transistor, the material must have a lowelectrical conductivity and a high breakdown field characteristic. Thus,an inorganic insulating layer, such as silicon oxide, having anelectrical conductivity of less than 10⁻¹² S/cm and a breakdown field ofgreater than 1 MV/cm, is widely used as a gate insulating layer.

However, inorganic insulating layers, which are formed at hightemperature, may affect other layer materials previously formed on asubstrate through preceding processes (to be called as pre-processlayers hereinafter).

On the other hand, organic insulating layers, which are formed at lowtemperature, do not affect pre-process layers. Thus, research intoorganic insulating layers as new gate insulating layers is beingvigorously conducted.

Known methods for fabricating organic insulating layers include a spincoating method and a monomolecular layer formation method using theLangmuir-Blodgett film process, which are both advantageouslysimplified, low-temperature techniques.

However, these techniques are effective only when they are applied to asmall-area substrate. As flat-panel displays tend to increase in area,these techniques are difficult to be applied thereto. Further, sincethese techniques are wet-type processes, a pre-process layer may bedissolved during the processes, limiting selection of the kind of thepre-process layer. Thus, it is quite difficult to design an organicsemiconductor transistor in various manners. Since the process forforming organic insulating layers is not in-situ performed with respectto the pre- or post-processes, the fabrication process and equipmentbecome complex, resulting in an increase in the fabrication cost.

Therefore, there is an increasing demand for methods for fabricating anorganic gate insulating layer on a large-area substrate with asimplified process.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a fabrication methodof an organic semiconductor transistor having an organic gate insulatinglayer by a low-temperature, dry-type process.

To accomplish the above object of the present invention, there isprovided a method for fabricating an organic semiconductor transistor,the method including forming a gate insulating layer using an organicpolymeric layer by vapor deposition which is a low-temperature, dry-typetechnique. First, a substrate is provided, and an organic gateinsulating layer is formed on the substrate by a vapor deposition methodusing organic monomer sources. Then, a polymerization reaction is causedto occur in the organic gate insulating layer to complete an organicpolymeric gate insulating layer.

Here, the organic gate insulating layer is preferably formed to athickness of 50 to 20000 Å.

Also, the vapor deposition method is preferably a vacuum depositionmethod.

The polymerization reaction is preferably thermal polymerizationperformed by a heat treatment at 100 to 400° C. or photo-polymerizationby light radiation of 150 nm to 10 μm.

The steps of forming the organic gate insualting layer and completingthe organic polymeric gate insulating layer using the thermalpolymerization are preferably in-situ performed.

The method may further include the step of forming an organicsemiconductor active layer on the organic polymeric insulating layer,after the step of completing the organic polymeric insulating layer, andthe steps of forming the organic gate insualting layer, completing theorganic polymeric gate insulating layer and forming an organicsemiconductor active layer on the organic polymeric insulating layer arepreferably in-situ performed.

In step of providing a substrate, an organic semiconductor active layerand a source/drain electrode are preferably formed on the substrate. Theorganic semiconductor active layer and the source/drain electrode arepreferably formed by the step in-situ performed with respect to thesteps of forming the organic gate insualting layer and completing theorganic polymeric gate insulating layer.

In the case where the organic polymeric gate insulating layer is apolyimide layer, the organic monomer source may include an aromatictetracarboxylic dianhydride monomer and an aromatic diamine monomer, andthe organic polymeric gate insulating layer is a gate insulating layer.In this case, the step of forming the organic gate insulating layer mayinclude separately evaporating the aromatic tetracarboxylic dianhydridemonomer and the aromatic diamine monomer so that the molar ratio of therespective monomers in the organic gate insulating layer becomes 1:1.

The steps of forming the source/drain electrode and the gate electrodeof the organic semiconductor transistor are preferably in-situ performedwith respect to the steps of forming the organic gate insulating layerand completing the organic polymeric gate insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and advantages of the present invention will becomemore apparent by describing in detail a preferred embodiment thereofwith reference to the attached drawings in which:

FIG. 1 is a representation of a method for forming an organic polymericgate insulating layer in a fabrication method of an organicsemiconductor transistor according to the present invention;

FIGS. 2 and 3 are cross-sectional views of methods for fabricating is astaggered-inverted type organic TFT (thin film transistor) according tothe present invention;

FIG. 4 shows a FT-IR (Fourier Transform Infrared Absorption) spectralresult for a polyimide gate insulating layer according to the presentinvention;

FIG. 5 is a graphic representation of the current-to-voltagerelationship of the polyimide gate insulating layer according to thepresent invention;

FIG. 6 is a cross-sectional view of a staggered-inverted type organicTFT according to the present invention; and

FIG. 7 is a graphic representation of the current-to-voltagerelationship of the staggered-inverted type organic TFT shown in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

A method for fabricating an organic semiconductor transistor accordingto the present invention will now be described in detail. However, thepresent invention is not limited to the following embodiments and isimplemented in various forms. Rather, these embodiments are providedonly so that this disclosure will be thorough and complete and willfully convey the scope of the invention to those who have ordinaryskills in the art. Throughout the drawings, organic TFTs areschematically drawn and thicknesses of various films are exaggerated forclarity. In the drawings, the same elements are designated by the samenumbers.

In the method for fabricating an organic semiconductor transistoraccording to the present invention, a gate insulating layer is formed ofan organic polymeric layer. A process for forming an organic polymericgate insulating layer is shown in FIG. 1.

Referring to FIG. 1, a substrate is provided to form a gate insulatinglayer (step 1). The substrate is used to form a transistor and may takeany form of a silicon substrate, a glass substrate or a plasticsubstrate. Also, a refractory plastic substrate having a high glasstransition temperature can be used as a substrate for an organicsemiconductor transistor. A large-area substrate of 8 inches or greatercan also be used as a substrate for a flat-panel display. A precedingprocess layer is formed on a substrate according to the kind of atransistor. For example, in the case of forming a staggered-invertedtype organic TFT, a gate electrode may be formed on the substrate. Inthe case of forming a staggered type organic TFT, an active layer and astacked layer structure of source/drain electrodes may be formed on thesubstrate.

In detail, the substrate is first loaded into a vapor depositionapparatus. Usable vapor deposition apparatuses include a vacuumdeposition apparatus.

Then, at least one organic monomer source is inserted into an evaporatorof the vapor deposition apparatus. In the case of forming a polyimidelayer as an organic polymeric gate insulating layer, it is preferredthat two kinds of sources are simultaneously used. A first source is anaromatic tetracarboxylic dianhydride monomer source. Usable examples ofthe first source include oxydiphthalic anhydride (ODPA), pyromelliticdianhydride (PMDA), benzophenone tetracarboxylic dianhydride (BTDA) andbiphthalic dianhydride (BPDA). A second source is an aromatic diaminemonomer source. Usable examples of the second source include materialsrepresented by the following formula 1:

Alternatively, a vinyl derivative monomer is subjected to vapordeposition and UV radiation to be polymerized, thereby forming a thinfilm having good insulating properties. The following reaction is causedto occur. Here, the properties of the thin film can be adjusted bychanging a substituent R in different. The substituent may be hydrogenatoms and may be necessarily the same with each other.

Then, an organic gate insulating layer is formed on the substrate (step2).

Physical parameters such as the degree of vacuum in a depositionchamber, a substrate temperature, power and so on, are controlled toform an organic gate insulating layer on the substrate according tocharacteristics of a vapor deposition apparatus used. When a vapordeposition chamber is used, the degree of vacuum is set to 10⁻⁶ Torr orless.

An organic gate insulating layer is called as such because most ofinsulating layers formed in the process exist in the phase ofmonomolecules or oligomers rather than polymers having completelyundergone polymerization.

If an aromatic tetracarboxylic dianhydride monomer source and anaromatic diamine monomer source are used, the respective sources areseparately evaporated and physical parameters are controlled so that themolar ratio of monomers in the organic gate insulating layer becomes1:1, thereby forming a film.

Finally, a polymerization reaction is caused to occur in the organicgate insulating layer, thereby completing an organic polymeric gateinsulating layer (step 3).

In step 3, polymerization is caused to occur in the organic gateinsulating layer formed in the phase of monomolecules or oligomers, sothat the organic gate insulating layer is turned into a polymeric film.That is to say, the organic gate insulating layer is subjected to a heattreatment at 100 to 400° C. to cause thermal polymerization or to lightradiation of 150 nm to 10 μm to cause photopolymerization, therebycompleting a gate insulating layer made of an organic polymeric film.

The thermal polymerization is performed by using a heater in a vapordeposition apparatus to raise the temperature of a substrate, therebyin-situ performing the steps of forming the organic gate insulatinglayer and completing the organic polymeric gate insualting layer.

The photopolymerization is performed by installing a light radiationunit in a vapor deposition apparatus, thereby in-situ performing thesteps of forming the organic gate insulating layer and completing theorganic polymeric gate insualting layer.

Now, a method for fabricating a staggered-inverted organic TFT accordingto an embodiment of the present invention will be described withreference to FIGS. 2 and 3.

FIG. 2 shows the step of forming a gate electrode 12 on a substrate 10.In detail, the substrate 10 with a shadow mask for defining a gateelectrode, is disposed in a vacuum chamber for gate electrodedeposition, and a metal for use as a gate electrode is placed in a metalboat. Usable gate electrode metals include aluminum having a low workfunction to realize a p-channel TFT. The vacuum chamber is maintained atthe degree of vacuum of 5×10⁻⁴ Torr or less, preferably 5×10⁻⁷ Torr, anddeposition is performed at a rate of 3-5 Å/sec, thereby forming the gateelectrode 12. The gate electrode made of aluminum, for example, isformed to a thickness of 1700 Å.

FIG. 3 shows the step of forming an organic polymeric gate insulatinglayer 14.

The substrate having the gate electrode 12 is placed in the vacuumdeposition chamber and monomer sources for forming target organicpolymeric layers are inserted into an evaporator such as a metal boat.

Subsequently, the vacuum chamber is maintained at the degree of vacuumof 10⁻⁶ Torr or less, preferably 5×10⁻⁷ Torr, and deposition isperformed at a rate of 5-10 Å/sec, thereby forming an organic gateinsulating layer having a thickness of 50 to 20000 Å.

In the case of using an aromatic tetracarboxylic dianhydride monomersource and an aromatic diamine monomer source, deposition is performedsuch that the molar ratio of the respective monomers in the insulatinglayer is 1:1.

Then, thermal polymerization is carried out for about 30 minutes toabout 2 hours by performing a heat treatment at 100 to 400° C., orphotopolymerization is carried out for about 10 minutes or less byradiating light of 150 nm to 10 μm, thereby completing the organicpolymeric gate insulating layer 14.

The organic gate insulating layer is formed to a thickness ofapproximately 1500 Å using 4,4′-oxydiphthalic dianhydride and4,4′-oxydianiline as an aromatic tetracarboxylic dianhydride monomersource and an aromatic diamine monomer source, respectively. Then, theorganic gate insulating layer is copolymerized in a vacuum oven atapproximately 220° C. for about 1 hour. FIG. 4 shows a FT-IR (FourierTransform Infrared Absorption) spectral result for the obtained organicpolymeric gate insulating layer according to the present invention. InFIG. 4, peaks are observed around 1379 cm⁻¹ (C—N), 1500 cm⁻¹ (C—C), 1720cm⁻¹ (C═O, asymmetric), 1778 cm⁻¹ (C═O, symmetric), from which thefinally obtained organic polymeric gate insulating layer is identifiedas a polyimide layer.

Also, it is confirmed that the resulting device consisting of thealuminum gate (1700 Å) and polyimide gate insualting layer (1500 Å) hasan electrical conductivity of approximately 10⁻¹¹ S/cm and a breakdowncurrent field of approximately 0.3 MV/cm, as shown in FIG. 5, which aresuitable characteristics so as to serve as a gate insulating layer.

Then, subsequent processes are carried out on the substrate having thecompleted organic polymeric gate insulating layer 14, thereby completinga staggered-inverted type organic TFT, as shown in FIG. 6.

In detail, an organic semiconductor active layer 16 is formed on theorganic polymeric gate insulating, layer 14 by a vapor depositionmethod, preferably a thermal evaporation method. The organicsemiconductor active layer 16 is formed of an organic semiconductormaterial, such as pentacene, oligo-thiophene, poly(alkylthiophene) orpoly(thienylenevinylene). The degree of vacuum in the vacuum depositionchamber is set to 5×10⁻⁴ Torr or less, preferably 5×10⁻⁷ Torr, anddeposition is performed at a rate of approximately 0.5 Å/sec, therebyforming the organic semiconductor active layer 16 having a thickness ofapproximately 1000 Å.

Then, the substrate 10 is overlaid with a shadow mask for source/drainelectrode and a metal material having a high work function is depositedby vacuum deposition, thereby forming a source/drain electrode 18. Here,gold is suitably used as the metal material. The degree of vacuum in thevacuum deposition chamber is set to 5×10⁻⁴ Torr or less, preferably5×10⁻⁷ Torr, and deposition is performed at a rate of approximately 3-5Å/sec, thereby forming the source/drain electrode 18 having a thicknessof approximately 1500 Å.

FIG. 7 shows the output characteristics of the organic TFT comprised ofaluminum gate (1700 Å), polyimide gate insulating layer (1500 Å),pentacene active layer (1000 Å) and gold source/drain electrode (1500Å). In FIG. 7, Id, Vds, and Vgs are drain current, drain-to-sourcevoltage, gate-to-source voltage, respectively. Referring to FIG. 7, thefield-effect mobility of the organic TFT is approximately 0.1 cm²N·s,which is quite a good level.

Although a method for fabricating a staggerd-inverted type TFT has beendescribed above in a preferred embodiment of the present invention, themethod of the present invention can also be applied to fabrication of astaggered type organic TFT having a structure in which an active layer,a source/drain electrode, a gate insulating layer and a gate electrodeare sequentially stacked. In this case, the respective steps of thefabrication method are in-situ performed. Further, the method forfabricating an organic polymeric gate insulating layer according to thepresent invention can also be applied to a general fabrication method ofan organic field-effect transistor.

INDUSTRIAL APPLICABILITY

As described above, in the method for fabricating an organicsemiconductor transistor according to the present invention, a gateinsualting layer is formed of an organic polymeric layer that can beformed at low temperature. Since the organic polymeric gate insulatinglayer is formed using a vapor deposition method, which is alow-temperature dry-type technique, it does not affect pre-processlayers.

Thus, pre-process layers can be freely selected and various organicsemiconductor transistors can be designed. Also, since the organic gateinsulating layer formation can be in-situ performed with respect toactive layer formation, and can be in-situ performed with respect togate electrode formation and source/drain electrode formation, thepresent invention has a significant advantage in terms of simplicity ofthe fabrication process and equipment. Therefore, organic semiconductortransistors suited to many applications can be easily fabricated at lowcost. Further, since the fabrication method according to the presentinvention employs vapor deposition, a gate insulating layer having goodproperties can be formed with a high level of uniformity, thereby makingit easy to fabricate a transistor on a large-area substrate.

1. A method for fabricating an organic semiconductor transistorcomprising the steps of: providing a substrate; forming an organic gateinsulating layer on the substrate by a vapor deposition method usingorganic monomer sources; and causing a polymerization reaction to occurin the organic gate insulating layer to complete an organic polymericgate insulating layer.
 2. The method according to claim 1, wherein theorganic gate insulating layer is formed to a thickness of 50 to 20000 Å.3. The method according to claim 1, wherein the vapor deposition methodis a vacuum deposition method. 4-5. (canceled)
 6. The method accordingto claim 1, wherein the polymerization reaction is photo-polymerizationby light radiation of 150 nm to 10 μm.
 7. The method according to claim6, wherein the steps of forming the organic gate insulating layer andcompleting the organic polymeric gate insulating layer using thephoto-polymerization are in-situ performed.
 8. The method according toclaim 1, further comprising the step of forming an organic semiconductoractive layer on the organic polymeric insulating layer, after the stepof completing the organic polymeric insulating layer.
 9. The methodaccording to claim 8, wherein the steps of forming the organic gateinsulating layer, completing the organic polymeric gate insulating layerand forming an organic semiconductor active layer on the organicpolymeric insulating layer are in-situ performed.
 10. The methodaccording to claim 1, wherein in step of providing a substrate, anorganic semiconductor active layer and a source/drain electrode areformed on the substrate.
 11. The method according to claim 10, whereinthe organic semiconductor active layer and the source/drain electrodeare formed by the step in-situ performed with respect to the steps offorming the organic gate insulating layer and completing the organicpolymeric gate insulating layer.
 12. The method according to claim 1,wherein the organic monomer source includes an aromatic tetracarboxylicdianhydride monomer and an aromatic diamine monomer, and the organicpolymeric gate insulating layer is a gate insulating layer. 13.(canceled)
 14. The method according to claim 12, wherein the step offorming the organic gate insulating layer includes separatelyevaporating the aromatic tetracarboxylic dianhydride monomer and thearomatic diamine monomer so that the molar ratio of the respectivemonomers in the organic gate insulating layer becomes 1:1, andcompleting the organic polymeric gate insulating layer includessubjecting the organic gate insulating layer to photopolymerization byradiating light radiation of 150 nm to 10 μm to yield a polyimide gateinsulating layer.
 15. The method according to claim 1, wherein thesubstrate is a heat-resistant plastic substrate.
 16. The methodaccording to claim 1, wherein the steps of forming the source/drainelectrode and the gate electrode of the organic semiconductor transistorare in-situ performed with respect to the steps of forming the organicgate insulating layer and completing the organic polymeric gateinsulating layer.